Why are you integrating [itex]y[/itex] from [itex]x[/itex] to [itex]1-x[/itex]? Are you told that [itex]y\geq x[/itex]? If not, you should be integrating from [itex]0[/itex] to [itex]1-x[/itex] instead.

right... so that's why I chose the integral limits for dy to be from x to 1-x.
Is it wrong?

Your response to gabbagabbahey's question was confusing. In your previous post, you answered, it was because you knew [itex]y \ge x-1[/itex]. The "x-1" suggested to me you somehow got that condition from the constraint [itex]x+y \le 1[/itex], and my point was that the constraint can only lead to [itex]y \le 1-x[/itex], the upper limit of the integral. There's no way you can get [itex]y \ge x-1[/itex] from it.

Even if you did have [itex]y \ge x-1[/itex], your answer didn't make sense. Why would the lower limit of the integral be x? Shouldn't it be x-1 if your inequality were true?